1. Field of the Invention
The present invention relates to a zeolite suspension used as a precursor for an interlayer dielectric film in next-generation semiconductor devices, a zeolite nano-crystal production method, a zeolite suspension production method and a zeolite thin film.
2. Description of the Prior Art
Zeorites are inorganic microporous crystalline materials. Their pore size is extremely uniform. Compared with conventional mesoporous silica with amorphous nature, zeolites have a better local order (ordered structure) and are therefore excellent in chemical stability, mechanical strength and heat conductivity. While aluminosilicate zeolites are hydrophilic, pure-silica zeolites are hydrophobic and adsorb little water.
As regards zeolites, there are the following prior art references in which MFI-type pure-silica zeolites are reported as being usable as excellent materials of low relative permittivity.    1. U.S. Pat. No. 6,573,131    2. U.S. Pat. No. 6,630,696    3. PCT-A2004-504716    4. JP-A2003-249495    5. JP-A 2004-79582    6. U.S. Pat. No. 6,533,855    7. U.S. Pat. No. 6,566,243    8. U.S. Pat. No. 6,660,245    9. U.S. Pat. No. 6,329,062    10. JP-A2004-149714    11. JP-A2004-153147    12. PCT-A2004-26765    13. Z. Wang, H. Wang, A. Mitra, L. Huang, Y. Yan, Pure-silica zeolite low-k dielectric thin films, Advanced Materials, 13 (10), 2001, pp. 746-749    14. Z. Wang, A. Mitra, H. Wang, L. Huang, Y. Yan, Pure silica zeolite films as low-k dielectric by spin-on of nanoparticle suspensions, Advanced Materials, 13 (19), 2001, pp. 1463-1466    15. S. Li, Z. Li, Y. Yan Ultra-low-k pure-silica zeolite MFI films using cyclodextrin as porogen, Advanced Materials, 15 (18), 2003, pp. 1528-1531    16. O. Larlus, S. Mintova, V. Valtchev, B. Jean, T. H. Metzger, T. Bein, Silicalite-1/polymer films with low-k dielectric constants, Applied Surface Science, 226 (1-3), 2004, pp. 155-160    17. Miki Egami, Semiconductor Device Porous Silica Thin Film Characteristics, Science and Industry, 77 (11), 2003, pp. 582-587
In prior art references 1, 3 and 13 above, Wang et al. disclose the results of in-situ growth of an MFI-type zeolite crystal film in a heat application atmosphere on a silicon substrate through immersion of the substrate in an autoclave coated with polytetrafluoroethylene in which starting chemicals of tetraethyl orthosilicate (TEOS), tetrapropyl ammonium hydroxide (TPAOH) and water have been introduced and well mixed. The film thus obtained is found to be excellent in mechanical strength (Young's modulus of 30 to 40 GPa) and have low relative permittivity of 2.7 to 3.3. While adaptability of the in-situ zeolite crystallization technique for the semiconductor fabrication technique leaves serious problems, the high potential of zeolite polycrystal thin films has been demonstrated in its principle.
Also in prior art references 1, 2, 3 and 14, Wang et al. disclose the results of the film formation by spin-coating using a conventional method on the synthesis of an MFI-type zeolite nanocrystal suspension by hydrothermal crystallization, in which after the synthesis a short-time low-speed centrifugation is used to remove crystals of large grain size, and the remaining suspension is applied onto the substrate by spin-coating. The film thus obtained has a bi-modal porous structure comprising micropores of 0.56 nm peculiar to MFI-type zeolites and interparticle mesopores of 2 to 17 nm. The mesopores are regulated to control the relative permittivity to a range of 1.8 to 2.1. Furthermore, y-cyclodextrin is used as a porogen (pore generator) to increase the mesopore volume fraction and succeed in materialization of an MFI-type pure-silica zeolite-based film having relative permittivity of 1.6 to 1.7. As the increased mesopores, it cannot be avoided to increase the hygroscopicity and lower the mechanical strength. Though Wang et al. have succeeded in obtaining a pure-silica film of low relative permittivity through the spin-coating of a liquid containing nanocrystal grains on a silicon substrate, no report is made of any experimental results proving the flatness of the film surface. They rather report that it is necessary to polish the surface of the film obtained through spin-coating using a liquid containing alumina particles for ten minutes or subject the film to secondary growth to acquire surface flatness. A check experiment was actually conducted in accordance with this report, the surface of the film obtained was in a terrible state, i.e. too coarse to obtain useful data through thin film measurement by spectroscopic ellipsometry.
Also, Wang et al. describe in prior art references 4 and 5 that improvement in surface flatness and pore diameter distribution could take place when mixing an organic substance or polymer into a liquid. However, no demonstrative data thereof is shown. While the internal pore surface of pure-silica zeolites is hydrophobic, the external surface of zeolite crystal grains and amorphous silica surrounding the grains have a silanol (—OH) group and consequently exhibit hydrophilicity. Taking this into consideration, Wong et al. have mixed methyl-group into a solution for application in a large amount of 50% by weight, thereby having succeeded in making the entire film hydrophobic.
According to the method of Gayner et al., as described in prior art references 6 to 9, a porous material of low relative permittivity that comprises two components, i.e. MFI-type zeolite crystals and a porous binder is obtained. The film obtained has relative permittivity of 2.15 to 2.45 and Young's modulus of 5 GPa or less. The porous binder is synthesized through addition of a small amount of acidic water to an ethyl alcohol solution of TEOS with hydrolyzed amorphous silica containing pores of 5% or less. However, they do not show any data on the surface flatness and pore diameter distribution of the film thus obtained.
In the method of Larlus et al., a porous irregular structure composite of MFI-type zeolite and acryl rubber polymer is used to materialize a film having relative permittivity of 2.0 to 2.56. Though they admit the film surface flatness, neither the mechanical strength of the film nor the pore diameter distribution thereof is touched upon.
Egami et al. reports a film of low relative permittivity having relative permittivity of 2.3 and Young's modulus of 8 to 10 GPa as described in prior art references 10, 11 and 17. The production method is directed to spin-coating of liquid containing MFI-type zeolite crystals. Followed by the spin-coating is the heat treatment at 400° C., and it is reported that in the subsequent Fourier transform infrared spectroscopy, the peak that ought to appear in the spectrum resulting from the MFI-type zeolite structure has disappeared. They also disclose a method for the production of a low-relative-permittivity thin film of hydrophobic amorphous silica (having relative permittivity of 2.5 or less and Young's modulus of 6 GPa or more). The method adopts TEOS as a silica source, TPAOH or tetrabutylammonium hydroxide (TBAOH) as a template molecule, methyl trimethoxysilane (MTMS) or methyl triethoxysilane (MTES) as a methyl source for promoting film hydrophobicity and ethyl alcohol as a solvent, and the reaction temperature and reaction time are as low as 50° C. and as short as 20 hours, respectively. A check experiment conducted under the same reaction conditions failed to detect zeolite crystals in the liquid. The film obtained is found to be a porous silica film though the synthesizing method is similar to the case of zeolite.
Takamura et al. disclose in prior art reference 12 that the mechanical strength of a low-relative-permittivity film of MFI-type zeolite is enhanced through the treatment with tetramethylcyclotetrasiloxane (TMCTS). However, the method of the production of an MFI-type zeolite described therein is the same as that of Wang et al, and the relative permittivity reported is 3.2 that is a rather high value.
In the prior art references, as described above, pure-silica films of low relative permittivity are obtained through spin-coating of the suspensions containing particles of various sizes. However, the thin films are formed each of MFI-type zeolite, and no data on thin film surface flatness is shown, meaning that the surface flatness would be insufficient. The hydrophobicity of the films cannot be said to be satisfactory. Furthermore, the synthesizing method in each of the prior art references is used to give rise to large particles and requires centrifugation prior to the spin-coating of a suspension containing MFI-type zeolite crystals, resulting in a cumbersome and costly production process.
The present invention has been proposed in view of the above drawbacks and has as its object provision of a zeolite suspension, a zeolite nano-crystal production method, a zeolite suspension production method and a zeolite thin film, in which it is possible to secure sufficient strength of a zeolite film even when the principal component comprises zeolite nano-crystals other than MFI-type zeolite crystals, materialize reduction in the relative permittivity thereof, enhancement of the surface flatness thereof and enhancement of the hydrophobicity thereof and simplify the production process thereof to render the cost thereof inexpensive.